In recent years there has been a significant growth in WLAN (wireless local network) due to the ever increasing demand of wireless communication products. Such growth is particularly obvious after the promulgation of IEEE 802.11 WLAN protocol in 1997. IEEE 802.11 WLAN protocol not only provides many novel features to the current wireless communications but also provides a solution of enabling two wireless communication products manufactured by different companies to communicate each other. As such, the promulgation of IEEE 802.11 WLAN protocol is a milestone of the development of WLAN. Moreover, IEEE 802.11 WLAN protocol ensures that core device is the only solution of implementing a single chip. Thus, it can significantly reduce the cost of adopting wireless technology so as to enable WLAN to be widely employed in various wireless communication products.
Conventionally, electromagnetic waves are susceptible of generation when a wireless communication product is transferring data in high power. And in turn, EMI (electromagnetic interference) may be caused by the electromagnetic waves. For solving problems associated therewith, many rules are promulgated by advanced countries. These rules impose a limitation on the import and use of wireless communication products found not complying therewith. In view of the above, the developers and manufacturers of wireless communication products have to take related rules into consideration in developing the control circuitry of the wireless communication product. Typically, a filter is provided after a power amplifier so as to cut out the generated harmonic spuriousness of high frequency. In such a manner, the produced wireless communication products are able to comply with the related rules. Conventionally, filters used in the production of the control circuitry of the wireless communication product are filters of high frequency made of ceramic material. The advantages of such filters of high frequency are that they can cut out harmonic of two or three times of a fundamental frequency and are much compact. The disadvantages thereof are expensive, requiring an additional installation procedure, cumbersome process, and much increased manufacturing cost. As an end, the production is low.
For solving the above problems, some manufacturers in the art use waveguide elements to simulate the desired filter circuit which is in turn employed to form a microstrip circuit 10 as illustrated in FIG. 1. Also, technology of PCB production is utilized in manufacturing circuit boards of wireless communication products in which the microstrip circuit 10 is formed on the circuit board. It is desired that the above configuration can effectively cut out harmonic spuriousness of at least two times of a fundamental frequency caused by nonlinear distortion of the power amplifier of the wireless communication product by means of the microstrip circuit 10 without using an additional filter of high frequency. However, the miniaturization of microstrip circuit contradicts the cut out range of harmonic spuriousness. As such, for achieving the purpose of cutting out a wider range of harmonic spuriousness (i.e., having a frequency of a larger number of times of a fundamental frequency) the produced microstrip circuit may be too large and complicated. As an end, it undesirably greatly limits applications of microstrip circuit, significantly increases the manufacturing difficulties, and compromises the goal of miniaturizing the circuit board and its product.
Harmonic of signals transmitted in high power is the most possible one that will not pass an EMI test conducted on a wireless communication product. This is particularly true for a signal having a harmonic of two, three, or four times of a fundamental frequency. Such is not acceptable.
Thus, it is desirable among developers and manufacturers of the art to provide wireless communication products complying with the related rules without greatly increasing the manufacturing cost and the size.